The major goal of the proposed research project is to investigate the processes by which human infants perceive speech and speech-like acoustic stimuli. Five general questions are addressed by this proposal. First, do infants perceive speech sounds in a specialized "speech mode"? More specifically, is the phnomenon of categorical perception mediated by a phonetic or an auditory (psychophysical) mechanism? Second, are the perceptual categories of speech (whether phonetic or auditory) present innately? Third, does early experience in a particular linguistic environment modify the perceptual categories of speech (whether innate or not)? Fourth, does speech perception in infants operate according to some process of feature analysis? And fifth, do infants show evidence of perceptual constancy in the processing of acoustically different speech sounds that are phonetically the same? These five general questions will be examined within the context of experiments covering four topic areas of speech perception: (1) voicing (2) place of articulation in stop consonants, (3) vowels, and (4) feature analysis and interaction. An operant had-turning procedure will be used to measure both discrimination and generalization to highly controlled synthetic and natural speech and speech-like stimuli using five- to ten-month-old infants as subjects.